GD3 is one form of sialosyl sphingoglycolipids; G stands for ganglioside (sialosyl sphingoglycolipid) and D stands for disialo. GD3, like other tumor-associated antigens such as GM2, GM3, GD2, and GT3, is known to be expressed on tumor cells such as those of human melanoma. The structural formula of GD3 is represented by NeuAc α2-8 NeuAc α2-3 Gal β1-4 Glc β1-1 Cer.
Previous reports regarding GD3 have revealed, among other findings, that development of a tumor correlates to expression of GD3, that GD3 is highly expressed on melanoma cells, and that administration of mouse anti-GD3 monoclonal antibody suppresses growth of tumors of melanoma patients. On the basis of these findings, GD3 has become of keen interest in immunotherapy of these types of tumors.
Thus, human immune response against GD3 is expected to provide beneficial effects during the clinical course of pathological conditions; in fact, a variety of clinical tests have been performed in the technical field of vaccines. However, successful results that fulfill the above expectation have yet not been reported (see, for example, Cheresh, D. A., et al., Proc. Natl. Acad. Sci., USA., 81, 5767-5771 (1984): Herlyn, M., et al., Cancer Res., 45, 5670-5676 (1985): Houghton, A. N., et al., Proc. Natl. Acad. Sci., USA., 82, 1242-1246 (1985): Livingston, P. O., Immunological Rev., 145, 147-163 (1995): Livingston, P. O., et al., Cancer Immunol. Immunother., 45, 1-9 (1997)).
Gangliosides, which have been found to be associated with tumors, are thus known to serve as a useful target in the immunological attack against cancer. However, they are acknowledged to have poor immunogenicity.
In order to overcome this drawback, there has been developed and disclosed a cancer vaccine composition for inducing or stimulating immune response of antibodies against gangliosides (Japanese Patent Application Laid-Open (kokai) No. 8-53366). This composition is an N-glycosylated product of ganglioside (i.e., N-glycosyl GM3).
Similarly, U.S. Pat. No. 5,102,663 discloses a 9-O-acetylated product of ganglioside (9-O-acetylated GD3).
Moreover, Japanese Kohyo Publication No. 8-508978 discloses that a similar cancer vaccine, GD3 complex vaccine (GD3-keyhole limpet hemocyanin complex), exhibits significantly improved antibody responses. According to the disclosure of this publication, a double bond in the ceramide backbone of GD3 are cleaved with ozone for chemical modification, thus introducing an aldehyde group, and the aldehyde group is caused to bind to the aminolysyl group of protein through reductive amination, to thereby construct a complex with a synthesized multi-antigenic peptide displaying repeats of malarial T cell epitopes, coat protein of Neisseria meningitidis (OMP), cationized bovine serum albumin (cBSA), keyhole limpet hemocyanin (KLH) and polylysin. That publication also describes that the most effective immunological adjuvant is QS-21, obtained through extraction of the bark of a tree found in South America and called Quillajasaponaria Molina (Aquila Pharmaceuticals, Worcester, Mass., U.S.A.: Kensil, C. R. et al., J. Immunol., 146, 431 (1991)).
Conventional vaccines prepared through employment of GD3 per se as an antigen exhibit only weak immune responses and their effects are transient. Moreover, they have drawbacks, in that the raw material GD3 is not readily available. That is, generally speaking, mass production of a desired ganglioside from a living organism is very difficult. Also, synthesis of a ganglioside through chemical synthesis or genetic engineering is very difficult.
When vaccines are prepared through a variety of chemical modifications of GD3, particularly in chemical treatment performed for improving antigenicity or in preparation of complexes, disadvantages are encountered in terms of intricate procedure in relation to procurement, preparation, synthesis, etc. of raw materials for antigens and complexes, and necessity for selection of immunoadjuvants.
Meanwhile, in recent years, molecular biological techniques have been employed in the technical fields of complex saccharides, and techniques for replacing the sugar chain with peptide are now under development. Previously, the present inventors have successfully obtained, from a phage display random peptide library through biopanning using a monoclonal antibody against a sugar chain, a peptide which exhibits specific binding to an antibody against one form of ganglyoside, GD1α.
This peptide (15 mer) has been found to mimic the sugar chain structure of a complex glycolipid (and thus is called a glyco-replica peptide), to be bound, with specificity to, a monoclonal antibody against glycolipid GD1α, and to inhibit binding of an antibody to antigen GD1α.
The present inventors have also produced this peptide through chemical synthesis, and have confirmed that the peptide reacts with a monoclonal antibody for GD1α, that a (chemically synthesized) replica peptide of GD1α inhibits cell adhesion of cancer cells of highly metastatic cancer cell lines, and that the replica peptide inhibits metastasis of cancer cells (Japanese Patent Application Laid-Open (kokai) No. 10-237099).
In addition, the present inventors have succeeded in obtaining, from a random peptide library, a peptide which exhibits to modulate glycosidase activity and reacts with specificity with an antibody against lactotetraosyl ceramide or lactoneotetrasyl ceramide (see Japanese Patent Application Laid-Open (kokai) No. 10-237098 and Saibo Kogaku, Dai Ishikawa and Takao Taki, 16 (12) 1821-1828 (1997)).
A recently published report discloses study results similar to the above-described ones obtained by the present inventors. Specifically, Qiu, J., et al. have disclosed in Hybridoma 18(1) 103-112 (1999) that a 15-16 mer peptide containing a Trp-Arg-Tyr sequence and obtained from a phage display peptide library through use of an antibody against a GD2/GD3 antigen exhibits cross reaction with the antibody.
Willers, J., et al. describe that they have obtained, from two phage display peptide libraries of phages displaying 15-mer and 8-mer peptides, four phage-displayed peptides capable of binding to anti-GD3 monoclonal antibodies MB3.6, MG22, and MG21 (Peptides, 20, 1021-1026 (1999)). According to this publication, these peptides were found to exhibit ability to binding to the anti-GD3 antibody employed for selection, and this binding ability was inhibited by GD3, but desired immunogenicity was not observed for any of the peptides.
An object of the present invention is to provide a novel peptide which mimics the structure of the sugar chain of ganglioside GD3 and exhibits high affinity with anti-GD3 antibody.
Another object of the present invention is to provide an immunogenic peptide capable of producing GD3-specific antibody; in particular, a peptide having a characteristic feature such that an antibody produced through immunization with an immunogen comprising the peptide cross-reacts with GD3, and therefore, has utility as a vaccine which replaces GD3.
A still further object of the present invention is to provide a DNA sequence coding for the above-mentioned peptide, a recombinant expression vector in which the sequence has been integrated, a host cell harboring the vector, and a recombinant expression product produced by the cell.
A still further object of the present invention is to provide a pharmaceutical composition containing as an active ingredient the above-mentioned peptide or the recombinant expression vector.